The present invention relates to fail-safe circuits for use in anti-lock braking systems and in particular to a circuit for use in conjunction with the output circuit of the electronic control unit of such a braking system to obviate erroneous operation of the system's modulating valves in response to electrical shorts.
Anti-lock braking systems are now well known. Typically, such systems incorporate a micro-processor based control unit which receives signals via appropriate sensors associated with the vehicle's wheels to determine the existence of an imminent wheel-lock condition during braking. Upon determining the existence of such a condition, the control unit generates control signals which are applied to solenoid operated modulating valves which automatically control the application of pressurized braking fluid to the vehicle's brakes. It is also typical for such systems to incorporate a power relay operatable in response to a signal from the control unit for supplying operating energy to the solenoid valves. It is further well known that a motor vehicle's electrical system, which includes such elements as a high voltage ignition, an alternator, and various relays and switches, generates a substantial number of spurious signals, noise and the like and that the components of the vehicle including the relatively sophisticated electronic components associated with an anti-lock braking system exist in a hostile environment which includes wide temperature swings, saltwater, pollutants and the like.
Since an anti-lock braking system is an integral part of the vehicle's braking system it is extremely important that the system be as nearly fail-safe as possible to obviate improper operation of the vehicle's brakes and further to assure that the system will shut down and restore, as nearly as possible, normal functioning of the vehicle's brakes in the event of a system failure.
One of the potential failures that can affect such an anti-lock braking system is the occurrence of short circuits in the wires or connectors associated with the control unit, energizing relay, and modulating valve solenoids, such problems being particularly acute when these elements are subjected to such media as saltwater or other corrosive and/or conductive pollutants. Under such circumstances, short circuits can create a situation simultating a control signal thereby resulting in false operation of the system and possible failure thereof. Accordingly, there exists a need to provide circuitry that will reduce the possibility of spurious operation of the anti-lock braking system's solenoid valves and/or energizing relay caused by saltwater or similar conductive and/or corrosive materials.
Broadly, it is a fail-safe circuit for preventing false operation of the solenoid control valves of an anti-lock braking system. The circuit is used in conjunction with the control circuit for the anti-lock braking system, the latter including a computing circuit for receiving signals indicative of the rotational behavior of the vehicle's wheels and generating a plurality of braking control signals in response thereto. The system further includes a plurality of braking pressure solenoid control valves responsive to the control signals for modulating the application of the pressurized braking fluid to the vehicle's wheels. The circuit includes a relay driver circuit which is connected to the competing circuit to generate an output signal of polarity opposite the polarity utilized to actuate the solenoid valves. An output connector is provided for the control unit and connects the driver circuit to the solenoid energizing relay and to the solenoid operated modulating valves. The other terminals of the solenoid operated modulating valves are connected to a source of operating energy through the contacts of the energizing relay whereby a short circuit in the connecting wires or the pins of the connector will disable one of the relay of the solenoid valves in response to short circuits which cause spurious application of operating voltage thereto and conversely will disable the other of the relay or the solenoids in response to a short ground.
It is therefore an object of the invention to provide a fail-safe circuit for an anti-lock braking system which prevents the false operation of solenoid control valves and energizing relay caused by short circuits in the connectors and/or wiring between the control unit and the relay and solenoids.
It is another object of the invention to provide such a circuit which will operate in response to a short circuit to a positive potential to disable one of the relay and solenoid valves and to a short circuit to ground to disable the other of the relay or the solenoid valves.
Still another object of the invention is to provide such a circuit which can be incorporated in an anti-lock braking system with minimum cost while simultaneously providing high reliability.